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<title>Inheritance</title>
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<center><h1>Inheritance.</h1></center>
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<p>
Inheritance enables classes to use the properties of other classes and
add to those properties.
<p>
The thing to remember about object programming is that it attempts to
create a software model as close to the real world as possible. Inheritance
forms an important role to meet this need. Take the following example.
<p>
A fabric can have a colour and size and it is also used to make clothes
as well as tents. Now looking at this from the other angle tents are made
from fabric and poles but clothes are made from fabric and buttons.
So, Tents and clothes have something in common, <blink>fabric!</blink> If we described
tents and clothes in C++, we can seperate fabric into a seperate class
that the Tent and Clothes classes can inherit.
<p>
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       -----------         
      |  Fabric   |        
       -----------         
          A    A           
          |    |           
       ---      ---        
      |            |       
 ---------     ----------- 
|  Tent   |   |  Clothes  |
 ---------     ----------- 


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<p>
Now for some buzz words:
<ol>
<li>Fabric is a <b>base class</b>
<li>Tent and Clothes are <b>derived classes</b>
</ol>
This is what the code looks like:
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// ... The <b>base</b> class 'Fabric'
// ... is no different to normal.
</pre>
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<pre>

  class Fabric
  {
  public:

       Fabric() {};
      ~Fabric() {};

      SetSize(int x, int y)
      {
          Length = x;
          Width  = y;
      };

      SetColour(char *C)
      {
         strcpy(Colour, C); 
      }

  private:
      int  Length;
      int  Width;
      char Colour[20];
  };

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// ... The <b>derived</b> class 'Tent'
// ... names 'Fabric' as a base class.
</pre>
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<pre>

// ... The <b>derived</b> class 'Clothes' also
// ... names 'Fabric' as a base class.
</pre>
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<pre>

  class Tent <font color=red>: public Fabric</font>
  {
  public:

       Tent() {};
      ~Tent() {};

      SetNumOfPoles(int P)
      {
          Poles = P;
      };

  private:
      int  Poles;
  };
        
</pre>
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<pre>

  class Clothes <font color=red>: public Fabric</font>
  {
  public:

      Clothes() {};
      ~Clothes() {};

      SetNumOfButtons(int B)
      {
          Buttons = B;
      };

  private:
      int  Buttons;
  };
        
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<p>

What we now have are three classes with the following public methods.

<ul>
<li>Fabric
<ol>
<li>SetSize
<li>SetColour
</ol>
<li>Tents
<ol>
<li>SetSize
<li>SetColour
<li>SetNumOfPoles
</ol>
<li>Clothes
<ol>
<li>SetSize
<li>SetColour
<li>SetNumOfButtons
</ol>
</ul>


<h2>Using the classes</h2>

So we have three classes, they are used in the same way as before but
with one extra feature.
<p>
Consider this piece of code.


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  void Init(Fabric &Material);

  main()
  {
      Tent       Frame;
      Clothes    Jacket;

      Init(Frame);
      Init(Jacket);
  }

  void Init(Fabric &Material)
  {
      Material.SetColour("Red");
      Material.SetSize  (10, 20);
  }
     
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<p>

The function 'Init' expects an object of type 'Fabric' but we
are passing objects of type 'Tents' and 'Clothes', this is OK as
'Tents' and 'Clothes' are derived from Fabric.

<p>
<hr>
<h2>Examples:</h2>
<img src="../../GRAPHICS/computer.gif" alt="o">
<a href="../EXAMPLES/inherit.cc">The final program.</a>
<p>

<hr>
<h2>See Also:</h2>


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<font color=brown>
<address><a href="../../address.html">Martin Leslie</a> 
29-Dec-97</address><p>
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